In this renewal application we seek continued support to test the overall model that fetal B cell development is generated during a distinctive hematopoietic process and proceeds by novel modes of selection. We will do this by continuing (and extending to earlier stages) our studies of hematopoiesis, comparing B/T/myeloid lineage restriction in clonal assays, and altering genes that may play a role in this process by retroviral gene transduction of sorted cell subsets (Aim 1). We will also continue efforts to identify and characterize genes differentially expressed in fetal and adult B lymphopoiesis, cloning and characterizing genes identified by representation difference analysis, identifying new genes by DNA array screening, and identifying surface proteins differentially expressed on B precursors using flow cytometric selection of phage display antibody clones (Aim 2). Finally, we will determine the reason for the different proliferative response by fetal pre-B cells to pre- B receptor assembly, distinguishing whether the difference is upstream or downstream of the syk kinase, assessing the contribution of different MAP kinase signaling pathways, testing the role of btk kinase in pre- BCR signaling, and investigating signaling by a chimeric ret receptor kinase transgene (Aim 3). These studies will provide a clearer picture of fetal and adult B lymphopoiesis and elucidate how modes of selection affecting developing B cells differ. These studies have implications in the area of stem cell transplantation, neonatal immunity, and the origin of B cell subsets. Specifically, one characteristic of fetal B cell development is preferential generation of CD5+ B cells, the surface phenotype commonly associated with chronic B cell leukemia. Finally, increasing our understanding of lineage commitment during hematopoiesis and the regulation of pre-B cell proliferation provides fundamental information that may generate insights into treatment of malignancies of early hematopoietic cells.